class Foo

  def list_reboots
    "foo1\nfoo2\nfoo3\n"
  end

  def analyse_reboots
    list_reboots.split("\n").map {|x| "--#{x}--"}
  end
end

class Tester

  def scenario1
    "boot1\nboot2\nboot3\n"
  end

  def do_test
    foo = Foo.new
    puts foo.analyse_reboots
    prepare(foo, :list_reboots, :scenario1)
    puts foo.analyse_reboots
    puts Foo.new.analyse_reboots
  end

  def prepare(obj, obj_method, with)
    with_method = method(with)
    obj_metaclass = class << obj; self; end
    obj_metaclass.__send__(:define_method, obj_method) {with_method.call}
  end
end

Tester.new.do_test




/tmp $ ruby ./replace.rb 
--foo1--
--foo2--
--foo3--
--boot1--
--boot2--
--boot3--
--foo1--
--foo2--
--foo3--